This invention relates to the reduction of xe2x80x9cdynamicxe2x80x9d and equilibrium surface tensions in water-based systems by using a combination of fluorocarbon and hydrocarbon surfactants.
In printing and coating applications, when thin water-based coatings such as inks, paints or varnishes are applied to a hydrophobic surface such as a polyolefin or polyester film, a surfactant is generally added to the coating material to lower its surface tension and make the surface easier to wet and coat smoothly. The suitability of the surfactant and the amount required is typically determined by measurements of the surface tension of the coating material after adding the surfactant.
If these measurements are made at equilibrium conditions, they may be grossly misleading for a coating application carried out at high speed, where new surfaces or interfaces are being rapidly created by the physical spreading of the coating on the substrate, or by pneumatic means such as spraying. Then there may be insufficient time for adequate surfactant diffusion, adsorption and molecular orientation at the coating-surface interface to reach equilibrium conditions. For such applications, the surface tension measured under dynamic conditions may be more meaningful.
In high-speed printing and coating applications, if the surface tension of the coating is insufficient to wet the substrate after a few seconds the coatings, and especially thin coatings, will xe2x80x9cde-wetxe2x80x9d resulting in undesirable surface defects. That is, an initially smooth coating can xe2x80x9ccrawl backxe2x80x9d or xe2x80x9cretractxe2x80x9d from the surface, and create an uneven, rippled surface appearance. In this case, the equilibrium surface tension measurement also becomes important.
The rapid reduction of surface tension of a water-based solution may also be important in other applications. For example, in agricultural applications, quick wetting of a leaf surface is important for retention of the agricultural spray solution being applied to a plant. Thus the efficient application of such sprays may be heavily dependent on the surface tension of the spray solution as a function of time. Many hydrocarbon surfactants are used in the coatings industry to promote good dynamic effects, that is, offer low initial surface tension. However, such surfactants generally do not offer the low equilibrium surface tension reduction needed to ensure consistent wetting of many substrates. In addition they may cause the appearance of surface defects in the coating because of relatively low solubility in waterborne formulations. Polymeric films may require pretreatment for surface modification by means such as corona treatment, flame ionization, or pre-application of a xe2x80x9ctie-layerxe2x80x9d or primer to the substrate.
Although fluorocarbon surfactants offer excellent equilibrium surface tensions, they are not widely used in such high-speed applications because, in addition to being costly, they generally do not give a low initial surface tension and may cause foaming.
An article by Hirt, et al., appearing in Colloids and Surfaces (1990), 44, pp 101-117, and titled xe2x80x9cDynamic Surface Tension of Hydrocarbon and Fluorocarbon Surfactant Solutions using the Maximum Bubble Pressure Methodxe2x80x9d, discloses that equimolar and 3/1 molar ratio mixtures of a nonionic hydrocarbon surfactant and a nonionic fluorocarbon surfactant synergistically improve the dynamic surface tension of an aqueous solution at an overall surfactant concentration of 30 millimoles per liter (mM). For an overall concentration of 3 mM, such synergy was found at lower bubble frequencies, but not at the higher frequencies desired for high-speed applications. At overall concentrations of 0.3 mM and 0.03 mM, no synergy was found. Thus, useful synergy was found only at relatively high overall surfactant concentrations of 30 mM, of which the fluorocarbon surfactant represented 15 mM in the equimolar mixtures and 7.5 mM in the 3/1 molar mixtures. At these concentrations, the fluorocarbon surfactant is costly and may cause foaming.
There is a need for a surfactant composition for coating and printing compositions which will reduce the initial or xe2x80x9cdynamicxe2x80x9d surface tension and equilibrium surface tension at lower fluorocarbon surfactant concentrations. The present invention provides such a composition.
This invention comprises a composition for use in a coating or printing composition to reduce surface tension quickly and substantially when the coating or printing composition is applied to a substrate surface. The composition comprises a hydrocarbon surfactant and a fluorocarbon surfactant, wherein the concentration of hydrocarbon surfactant is from about 0.01% to about 0.5% by weight and the concentration of the fluorocarbon surfactant is from about 0.0001% to about 0.3% by weight, provided that the concentration of hydrocarbon surfactant is less than or equal to 0.08% by weight when the concentration of the fluorocarbon surfactant is greater than or equal to 0.1% by weight. The composition reduces the surface tension of a coating or printing composition to which it is added. The reduction in surface tension is both initial and quick (for example within ten seconds) after new surfaces/interfaces are created. Preferably the concentration of fluorocarbon surfactant is from about 0.0005% to about 0.3% by weight. More preferably the concentration of fluorocarbon surfactant is from about 0.001% to about 0.1% by weight.
This invention also comprises a method for reducing surface tension of a coating or printing composition when applied to a substrate surface comprising addition of the above described surfactant composition to the coating or printing composition. This invention further comprises a method for reducing surface tension of a coating or printing composition when applied to a substrate surface comprising the addition to the coating or printing composition of a composition comprising from about 0.0001% to about 0.3% by weight of a fluorocarbon surfactant, provided that the coating or printing composition contains a hydrocarbon surfactant and said hydrocarbon surfactant is present at less than or equal to 0.08% by weight when the fluorocarbon surfactant is present at greater than or equal to 0.1% by weight.